A blocking mix refers to the potting soil being used to create soil blocks. I would refer to it as blocking mix versus potting soil. Not all potting soils are the same, and more often than not, you do have to alter them. This is really no big deal and a lot of people are turned off to fixing a bought product, or creating a special recipe with obscure ingredients like rock dust, or technical terms like compost and garden soil. I understand completely....
One time, I ran out of my favorite ingredients. I had a lot of blocks to make that day, and the nearest garden center was 100 miles away. Add to that, a lot of my ingredients come from no where else, except California. I bought some cheap generic potting soil from our local hardware store. My results were dismal compared to my own. The blocks just wouldn't stay formed, they couldn't handle being saturated and compressed. Other times, replacements have dried out too quickly. Sometimes, they would stink due to the composted cow manure. And, some soils are way too chunky to fit in the Micro 20(3/4" blocker). I just wanted to make blocks that day, not mix a batch of my own stuff.
For our readers who "just want to make blocks", and "just want to buy some potting soil at the local garden center", here is my advice. First, you do have choices these days, but it pays to READ THE INGREDIENTS. You are looking for a soil that is actually soil-less. I know our home made recipes calls for soil as the secret ingredient, but I KNOW my soil. Your looking for mostly Peat Moss, some perlite or vermiculite, and a little compost. Compost again is our secret ingredient, but NOT animal compost, as it might stink, it might not be from organic animals, and it might not be thoroughly composted. Compost in our recipe is from our own home made vegetable or "green and brown" compost. Still, most bagged potting soils have composted bark or forest litter, and that can be too chunky. Watch out for excess fertilizers or nitrogen contents above the 2 range in the analysis(eg. 2-4-1, 2 being the N). Excess nitrogen can and will inhibit seed germination. Also, look for "horicultural or plug grade" ingredients, that means they're smaller particles, easier to use. So, go ahead and learn about the potting soils before you actually buy them.
Next, buy small bags first, and do a sample run. Make up some blocks and see if they hold up. See if the potting soil is finely screened or not. If it's not, you'll know. It will clog the block makers. You'll have to go to the hardware store and purchase some 1/4" hardware cloth, or screen. Build a nice wood frame for it, about a 2'x 3', and sift out all the chunks and sticks and bark and big pieces of perlite. You'll enjoy having a soil screen around, anyway. But, if that's not what you want to do either, buy a couple of brands in a small bag, test them, take back what you don't like. Stores are used to this, and if you didn't use too much, explain why you don't want it, and they'll readily take it back. (By now, I think you're getting the hang of it!)
Finally, the final analysis comes from the actual germination rate. Please use fresh, new seeds from a reputable company to test your potting soil out. This ensures a good test, as the seeds will be closer to 90% germination rate and above. Make sure to read the instructions on the seed requirements and meet those specifics. When seeding blocks, you have three options: 1.) No cover; 2.) Cover with potting soil dust(either sift some or pinch out small particles of soil and sprinke over the seed holes); 3.) Cover with black plastic, like a garbage bag, and check back in 2 days and keep checking daily for sprouts. If, for some reason seeds aren't sprouting in a timely matter, check for green algae growing on your cubes. This is an indication of excess nitrogen. Note: Most blocks will grow green algae on them over time, and that is just fine. The algae will act as a tiny green manure and will break down as soon as the block is buried or transplanted. It's the rapid formation of algae that signals excess nitrogen. Read your label again, and determine if there really is too much fertilizer for proper germination.
Some other things to consider when testing out your products: Do they dry out too fast? Do they allow water to drain quickly? Do they crumble when handled? Are your blocks not perfect like I said they should be? They can be, you know? Remember, there is a learning curve in block making, and chances are, you are going to be the only one who knows what to do. Keep searching for the holy pail of blocking mix!
If you really want to know the secret to my Old Farm Boy Blocking Mix, I'll tell you.
It's made with 1/2 to 1/3 coco peat with peat moss. The coco peat prevents drying and the peat moss knits the block together. Then, I have replaced perlite with pumice stone or diatomite rock. Next, I replace compost and soil with
the same volume of worm castings. Finally, I add glacial rock dust, and sometimes Zeba Quench. There it is: The secret is out. Now go on and make some!
We are beginning an exhaustive study of every potting soil available in the U.S. and Canada. When this is ready for publication, we hope it will save gardeners time and money and get them "potting on" with ease and joy.